1. Field of the Invention
This invention relates to an antenna device for detecting a direction of an object which lies in a low angle direction.
2. Description of the Background
As the system for detecting a direction of an object lying in a low angle direction, there has already been known the "fixed beam system" as disclosed in the following literature: "Low-Angle Radar Tracking in the Presence of Multipath" by W. D. White, IEEE Transactions, Aerospace and Electronic Systems, Vol. AES-10, pp. 835-852, November 1974.
In the following, explanations will be given as to the fixed beam system in accordance with the above-quoted literature.
FIG. 9 of the accompanying drawings is an explanatory diagram indicating a positional relationship between the antenna device and an object to be detected. In the diagram, a reference numeral 1 designates the antenna device, a numeral 2 refers to the object to be detected, a reference numeral 3 denotes the surface of water, a numeral 4 represents a mirror image of the object 2 formed by the surface of water 3, a reference numeral 5 denotes a propagation path of a direct wave, and a numeral 6 refers to a propagation path of a reflected wave.
Assuming that the antenna device 1 is capable of forming two kinds of radiation beams in different shapes, that is: beam F.sub.A and beam F.sub.B, they satisfy the summetry condition as represented by the following equation (1) in respect of an arbitrary angle u. ##EQU1##
Further assuming that the axis of the antenna device 1 is oriented in the bisector angle of the object 2 and the mirror image 4, a received voltage V.sub.A due to the beam F.sub.A can be represented by the following equation (2): ##EQU2## (where:
Es denotes amplitude of the direct wave;
u represents an incident angle of the direct wave into the antenna device 1;
-u represents an incident angle of a reflected wave into the antenna device 1;
.rho. denotes a voltage reflection coefficient of the surface of water; and
.phi. indicates a phase difference between the direct wave and the reflected wave).
Similarly, a received voltage V.sub.B due to the beam F.sub.B can be represented by the following equation (3): ##EQU3##
Taking a ratio between V.sub.B and V.sub.A, it may be represented as follows: ##EQU4##
The above equation (4) may be arranged by substituting the symmetry condition in the equation (1) to be as follows: ##EQU5##
As the consequence of this, by the measurement of the voltage ratio V.sub.B /V.sub.A, it becomes possible to find out an accurate direction u of the object 2 by use of a known function F.sub.B /F.sub.A, without having regard to existence of the reflected wave.
As described in the preceding, the fixed beam system is effective as to detecting the direction of an object lying in the low angle direction, while it has a point of problem such that the method of constructing the antenna device satisfying the symmetry condition of the above equation (1) was not known.